Prefractionation of hydrocarbon alkylate to eliminate propane



DCC. 28, 1948. I* KNlEL PREFRACTIONTION OF HYDROCARBON LKYLATE T0ELIMINATE PROPANE Filed April 29, 1942 Patented Dec. 28, 1948PREFRACTIONATION OFHYDROCARBQN. ALKYLA'IIE T0 ELIMINATE PRLQPANE Ludwig`KnieL, Jackson Heights, N. Y., assignor to. The Lummus Company, NewYork, N. Yl, a;

corporation of Delaware Application April 29, 1942,. SeriaLNd.. 440,931

isoparafnic reactant from the alkyiation,system,y

particularly in the sepa-ration of; the hydrocarbon mixture dischargedfrom the alkylation zone for the recovery oil the desired, alkylate.This problem primarily involves, fractionation difliculties in thehandling oi relatively large `amounts of reactants and unreactivematerials utilized in the ormationo a, relatively small amount ofdesired alkylate with the consequent large equipment, required toaccomplish the necessary Ltra'ctionation.

Since it is necessary to have the isoparafnic reactant, e. g.,.isobutanavpresent. in an 'amount in substantial excess off that requiredto react with the olenic reactant, e. g., a butene, in the alkylationzone in order toy obtain a` substantial degree of alkylation, it becomesdesirablefirom anec'onomic viewpoint tcrecycle the excess isoparainicreactant separated from the alkylate to the alkylation Zone to maintainthe desired ratio ot isoparain. to oleiin therein. At. the same time,however, the recycling of hydrocarbons lower boiling than theisoparaffinic reactant, e.` g., propane, which is invariably introducedinto the alkylation system along with the C4 fraction containing thevisobutane rand/or the `butenavis to be avoided as much as possiblesince, for optimum operation, itis necessary to maintain a lowconcentration of propane in, the, alkylation' zone.

Accordingly, itis an object oi myr invention to provide animprovedmethod of' fractionating, the hydrocarbon mixture discharged from thealkyl'af tion- Zone in, an isoparain-oleiin alkylation systemwhereby amaterial saving in equiignfnent and operating costs can be efected.

It isa further object ot. my invention to provide an improved method of,separating the isobutane and the propane from, the hydrocarbon eiiluentfrom the' alkylation zone in an isobutanebutene alkylation system forrecyclingof the isobutane to the alkylation zone and elimination cil thepropane from. the alkylati'on. system.

It. is another objectof my invention to provide 260'-683.4`)` 5" 2 aconvenient method of eliminating undesirable propane from anisobutane-buteneY alkylation system, Without, the use of, excessive,fractionating equipment and'with only a.` negligible loss of'valuableisobutane. Y;

Further objects` and, advantages of' my. invention Willv be apparentfrom the following description of' apreferred form of? embodmentthereoftaken in connection. with. the accompanying drawing, which represents atypical flow sheet or. a process embodying my irwention.`

For convenience only, my invention will be. described, in connectionwith. the alkyl'ation of isobutane with a butene in the presence ofconcentrated sulfuric acid. to. form high antil'mocl` gasolinecontaining a major proportion of' isooct anes. It. .will be.appreciated, hoxever,4 that my invention is not. so` restricted but isapplicable to, the alkylati'on of anyy `isoparailin with any olefin,Whether such materials. are, normally gas.- ecus or not, inthe presenceof` any suitable alkylation catalyst to form predominantly isoparaffni'chydrocarbons suitable for use as a high antiknock motor fuel..

According to my invention. isobntane. and bntene, in the desiredproportions, containing propanev unavoidably admixed" therewith,arealkylated in the, presence of.` concentrated' sulfric acid to. form highantiknock hydrocarbons boiliing. within the range of gasoline. Duringthe alkylationrea-ctionthe isobutane is maintained' in excessV of, thatrequired to react with the` butene. The` resulting alkylate product isseparated from the acid and is neutralized,A and.` the excess iso.'-vbntane and lower boiling hydrocarbons including propane are separatedas overhead from the allwlate in. aprefractionating step. The isobutaneoverheadlcontains the'propane in a higher concentration than at any'other point in" the alkylation, system., and the propane is concentratedina stream of relatively small. volume, at a. poi-nt in the alkylationsystem` Where it can bemost. readily eliminated therefrom by customaryfractionating, operations, This overhead stream subjected to a furtherfractionation step to, separate the propaneirom the isobutane. Thepropane is eliminatedifrom the; system, and theisobutane is recycled tothe alkylation zona, In. accordance with my inventiom a C4 frac'.- tionci normally gaseous hydrocarbons, such as a. butanecut obtained fromnatural gas or from the. stabilization oi natural. gasoline or a C4 outof.. cracking still gases or, the like is introduced through line i0into desobutanizer towerml'z. This feed material contains isobutane,normal ing amounts in both the overhead andthe bottoms streams accordingto the particular fractionating con-ditions. The isobutane and the lowerboiling hydrocarbons are taken Volif-'overhead through line i6 forcondensation in condenser I8. The resulting condensate is collected inaccumulator '20, from' which a portion of the condensate is returnedthrough line 2| to tower l2 as reflux.

vThe remainder 4of the isobutane condensate.

from accumulator 20 is passed through line 23 to the alkylation reactoror chamber 25,'wherein alkylation of the isobutane with the butenes iseffected in the presence ofconcentrated sulfuric acid. Butenes maybedesirablyintroduced into reactor 25 through line 26 from a suitableoutside source of supply. The molal ratio of the isobutane to thebutenes introduced into `reactor 25 should be at least 1:1 andpreferably within the range of 2:1 to 10:1 in order to accomplish a.substantial degree of alkylation. 'I'he excess isobutane may be obtainedby recycling excess isobutane within the system orv by introducingadditional' isobutane through line 23 from some extraneous source. Thesulfuric acid, which acts as a catalyst for the alkylation reaction,should have a concentration of between 90 and 100% and preferably ofabout 95 to 98%. The acid may be supplied to reactor 25 as through linesand 30. L

In reactor 25 the isobutane and the butenes are brought into v intimatecontact with each other and with thesulfuric acid to effect the desiredalkylation. The pressure within the reactor should be suflicient tomaintain the isobutane and the butenes in liquid condition at theparticular reaction temperature, which should be within the range of 0to 100 F. and preferably within the range of 40 to 60 F. The ratio ofacid to hydrocarbon is desirably maintained within'the range of l/to11/2 volumes of acid per volume of hydrocarbon feed to the reactor. Y

Reactor 25 is desirably provided with an agitating mechanism 32 toprovide the necessary intimacy of mixing. Any suitable mixing devicewhich providesa sufficient degree of agitation may be employed. Thelisobutane-butene mixture is maintained in reactor 25 for sufcient timeto effect substantial alkylation of the isobutane with the butenes;Substantially complete consumption of the butenes can be effected underthe proper alkylating conditions, which are Well known to the art.

The acid-hydrocarbon mixture from reactor 25 is continuously withdrawnthroughline 34 into settlingY chamber 36, wherein the mixture separatesinto an upper hydrocarbon layer and, a lower acid layer. This acid layeris withdrawn through 4line 3l, and the greater portion thereof ispreferably recycled through lines 38 and 30 to reactor 25 to effectalkylation of further isobu-l tane and butenes. A'portion of the usedsulfuric acid from settling chamber 36 may be continuously dischargedfrom the system through line 39, and fresh make-up sulfuric acid may becontinuously introduced into the system through line40.

The upperhydrocarbon layer in settling chamber 36 `is withdrawn throughline 42 and is passed into neutralizer 44. In this chamber thehydrocarbon mixture is treated with a neutralizing agent, e. g., analkaline solution such as a dilute aqueous caustic soda solution, whichis introduced into this chamber through line 45. The neutralizing agentservesl to neutralize the acidity of the hydrocarbons lresulting fromcontact thereof with the acid in reactor 25. The alkaline solutionseparates out in the bottom of neutralizer 44 and is continuouslywthdrawn therefrom through line 46.

According to my invention, the neutralized hydrocarbon mixture iswithdrawn from the top of neutralizer 4-4 through line 48 and is passedinto a prefractionating tower 50. This hydrocarbon mixture comprises thealkylate formed in reactorv 25, the excess isobutane, the normal butaneintroduced into the system along with the isobutane and the butenes, andany propane unavoidably admixed therewith. This hydrocarbon mixture ispreferably preheated in a suitable heater 5| before introduction intoprefractionator 50. Tower 5 0 is operated under suchv conditions thatsubstantially all the propane and 'a substantial proportion of the'isobutane are separated from the remainder of the hydrocarbon mixture.Preferably, theA fractionating conditions are such that only a minimumamount or substantially'none of the normal butane is separated from thehydrocarbon mixture'V along with the isobutane and the propane. Thepurpose of this prefractionation treatment is to concentrate the propanein avstream of relatively small volume at a'point inthe alkylation.system where it can be most readilyeliminated therefrom byfractionation.4

The separated isobutane and propane are taken off overhead through line52 forcondensation in condenser 54. The resulting condensate iscollected in accumulator 55, fromv which' a sufficient portion of thecondensate is returned through line 5B to tower- 50 as reflux. Theamount of reflux necessary to leffect this prefractionation of propane'and isobutane is surprisingly small, and a reflux ratio of about 1:1 to1.5:1 (ratio of reflux to net overhead product) is usually sumcient. .f

Further in accordance with my invention, the remainder ofthe condensateYfrom accumulator 55 is passed through line l58 to depropanizer 60,wherein the propane is separated from the isobutane admixed therewith.The separated isobutane is removed fromtower '60 as a bottoms streamthrough line 6| and is desirably admixed with the C4 hydrocarbon chargein line 23 passing to reactor 25. In this way a portion of the excessisobutane lis recycled to reactor 25 to maintain the necessary excess ofisobutane vtherein. The propane is removed as the overhead stream fromtower through line 62 for condensation in condenser G3, the condensatefrom which is collected in accumulator 64. This cona densation iscarried outto provide only suflicient reflux for tower 60, which refluxis supplied to rtower 60 through` line 65. The uncondensed proacer,

portion of the propane-isobutane stream in line 58 may be passeddirectly to reactor 25' through line Blaccording to the amount ofpropane that must be eliminated.

The bottoms stream from prefractionator 50 is passed through line 68into debutanizzery tower 10. This -bottoms stream comprisesv thealkylate admixed with some isobutane and the normal butane. Indebutanizer tower T the alkylate. is separated from the isobutane andAthe normal butane, which are removed overhead through line l2 forcondensation in condenser 13. The resulting condensate is' collected inaccumulator 1'4, from which asuicient portion of the condensate isreturned through line 15 to reflux tower 110. rPhe remainder of thecondensate is recycled through line 16 to deisobutanizer towerV |12vvfor separation of the isobutane from the normal butane. Theisobutanethus recovered supplies some of the excess isobutane requiredin reactor 2-5. If desired, line 11 may beprovided for recycling of aportion or all of the isobutane-in line ltv directly` to the reactorwhen the percentage of normal butane in this iscbutane stream isrelatively small.

The alkylateis. removed from; the` bottom of debutanizer tower throughline 1,3, intok alkylate ractionator Bil., If desired, the ractionatingconditions in tower 10 may `be maintained in such fashion that a portionof the butane is included in the alkylate bottoms stream to4 give the'resulting motor fuel fraction any desired volatility. I'n tower 8l!thealkylate is fractionated under conditions suitable ior production ofthe desired high antiknock gasoline, which is4 removed overhead throughline- 8l for condensation in condenser 82. A portion of the condensateis returned through line 83 as reflux for tower 80. The remainder of thecondensate is Withdrawn from the system through line 84 and comprisesthe desired high antiknock aviation grade motor iuel. The higher boilingalkylate product is removed from the bottom oi tower 8u through line 85for disposal as desired.

In operation, it is generally desirable to maintain the proportion ofpropane in the reactor 25 below 3% by weight since the propane is inertas far as the alkylation reaction is concerned and since it alsoadversely affects the yield of alkylate if it is present in too great aproportion during alkylation. Accordingly, unless the propane iscontinuously vented or eliminated from the alkylation system in somefashion, it will be continuously recycled along with the isobutane andwill gradually accumulate in the system and thereby adversely alect thealkylation reaction. It should be noted that the amount of propanevented through line 6B controls all of the fractionating operationsprior to this point.

My invention provides a method of concentrating the propane in a streamof relatively small volume at a point in the alkylation system where itcan be most readily eliminated. If the hydrocarbon eiiluent stream inline 48 is 'fractionated in one step solely for the purpose of removingthe propane overhead, a relatively large tower will be necessary.Furthermore, a relatively high reflux ratio will be required in order toprevent a substantial amount of isobutane from being carried overheadwith the propane and to prevent loss of isobutane. In addition, asubstantial supply of heat will be necessary in order to provide therequisite stripping eilect to strip the relatively small amount oipropane from the relatively large amount of bottoms.

Wilt-h my invention, these disadvantages, can be avoided, and a superiorseparation canfbe ob.-` tai-nedi at the same time. lThe fractionation inpretractionator 5B1 is carried out such that a substantial proportion ofthe isobutane is carried ofi overhead with the propane and onlysufficient refluxA is required to prevent the removal oi any normalbutane in the overhead stream. In this manner, the propane isconcentrated at a point inthe alkylation system where it can be mostreadily eliminated therefromr in a stream having a total volumelconsiderably-less than the volume of the hydrocarbon eilluent in line48. Accordingly. the easeoi` separation of the propane from theisobutane indepropanizer tower 5B materiallyy reduces the heatingloadand the other operating requirements necessary for the elmination ofthe propane from the system. The stripping of, the propane from theisobutane in tower Sr is considerably less diicult because of the muchsmaller amount of material being handled in the elimination or thepropane. Furthermore, the amountof isobutanelost with the eliminatedpropane is reduced to asubstantially negligibleminimum.

It will be appreciated that my invention is not necessarily limited tothe use of concentrated Suluric, acid as the alkylation catalyst. Othersuitable substances for electing this reaction comprise aluminumchloride, phosphoric acid, hydrouoric acid,l and the like. y

Although I have described` a preferred' i'orm o i embodiment of myinvention, I am aware that modifications.. may. be madey thereto;therefore, only,I such limitationsasappear in the claims, ap- Qeldedrhereinafter should be made.

I claim:

1. The process oi alkylating an isoparan with a normally gaseous olefinto form high antiknock motor fuel, which comprises fractionating a feedstock containing the isoparafl'in and unreactive hydrocarbons bothhigher boiling and lower boiling than the isoparafn to separate theisoparaiiin and the lower boiling hydrocarbons as overhead therefrom,contacting the isoparain overhead with the normally gaseous olefin inthe presence of an alkylation catalyst to effect alkylation of theisoparaifin with the oleiin, maintaining an excess of iso-v parain overthat required to react with the olen during alkylation, separating theresulting hydrocarbon mixture from the alkylation catalyst, subjectingthe hydrocarbon mixture to a prefractionation to separate the lowerboiling unreactive hydrocarbons and a portion only of the excessisoparaflin as overhead therefrom to concentrate the unreactivehydrocarbons in such overhead, passing such overhead to a zoneindependent of the Zone of said prefractionation and there fractionatingsuch overhead alone under fractionating conditions independent ofiractionating conditions within the prefractionation zone to separatethe unreactive lower boiling hydrocarbons from the excess isoparantherein, eliminating the separated unreactive hydrocarbons from thesystem, fractionating the remainder of said hydrocarbon mixture toseparate the remaining isoparaiiin from the alkylate, recycling theisoparafn recovered in the system to the alkylation zone to maintain theexcess of isoparaiiin therein, and iractionating the alkylate to recovera high antiknock motor fuel.

2. The process of alkylating isobutane with a butene to form highantiknock motor fuel, which comprises fractionatinga C4 fractioncontaining propane unavoidably admixed therewith to separate theisobutane and propane as overhead from the normal butane, contacting theisobutane overhead with a butene in the presence of concentratedsulfuric acid to eiect alkylation of the isobutane with the butene,maintaining an excess of isobutane over that required to react with thebutene during alkylation, separating the resulting hydrocarbon mixturefrom the sulfuric acid, heating the hydrocarbon mixture and subjectingit to a prefractionation to separate the propane and a portion only ofthe isobutane as overhead therefrom and to concentrate the propane insuch overhead, supplying only sulicient reflux to the prefractionatingzone to Yprevent separation of hydrocarbons higher boiling than theisobutane, passing such overhead to a zone independent of saidprefractionation zone and there fractionating such overhead alone underi fractionating conditions independent of fractionating conditionswithin the prefractionation zone to separate the propane from theisobutane, eliminating the propane from the system, fractionating theremainder of said hydrocarbon mixture to separate the remainingisobutane from the alkylate, recycling 4the separated isobutane to thealkylation zone to maintain the desired excess of isobutane therein, andfractionating the alkylate to recover a high antiknock motor fuel.

3. In the process of alkylating isobutane with a normally gaseous olefinin the presence of concentrated sulfuric acid to form high antiknockmotor fuel wherein the isobutane is admixed with propane and wherein theisobutane is present in excess of that required to react With the yzoneto separate the propane from the isobutane admixed therewith,eliminating the propane from the system, recycling the separatedisobutane to the alkylation zone, and vfractionating the remaininghydrocarbon` eiiluent to recover a high antiknock motor fueLthe reiluxratio in the prefractionating zone being maintained between about 1:1and 1.5:1.

`- Y LUDWIG KNIEL.

REFERENCES CITED The Vfollowing references are of. record in the le oflthis patent:

UNITED STATES PATENTS Number Name Date 2,172,560 Kemp et al Sept. 12,1939 `2,281,248 Putney Apr. 28, 1942 2,283,603 Goldsby et al May 19,1942 2,286,504 Parker June 16, 1942 2,338,272 Westenberg Jan. 4, 1944Gardner Apr. 4, 1944

